I will be honest, I beat the game and I did not see the point in making the effort to get those black coins for the hidden monster shop. I was quite disappointed on what that shop offered and would not recommend sweating it too much to get those coins. I thought there would at least be some cool armor or something. I guess the heal ring sounded cool but I had so much money I just had endless MP healing items which can just be purchased normally with much less hassle. Plus money is not a huge issue, especially at the end of the game.
I also did not see much use of the negotiation level (mine was at 10 by the end of the game) but maybe it had some hidden benefits I missed. I guess it was easy to run from battle? I never had a monster actually help me though even though friendship with some of them was quite high. I got a few interesting items too, here and there, that would sometimes be useful for a bit but everything the game provides for you in the end game surpasses any of those items from what I could tell. Also, for me, the best damage vs bosses was casting "strength up" and just attacking normally. Maybe lucifer did slightly more damage than an attack and maybe one other summon was useful. There were 1 or 2 multi hit spells I used for random encounters.
Overall the hacking and translation work were top notch but I did not like the non-descript names for spells and some items. Especially spells, it was just a trial and error kind of thing to see what they actually do so maybe the names can be improved and descriptions added.